Pic 5 shows why it hasn't slid off and for some reason not mentioned over the years...keying points...and they called themselves researchers!

Other than...Trethevy had a HUGE scare in the past and is not now as it is was then prior to that scare. The only mystery left is why all the antiqueries of the past have not sussed it out.

Roy , apart from the two blokes going on about the light coming through the hole and various other points , who were these "researchers " and what was there to "suss out " ?

Researchers was probably the wrong word to use George, but those that have written up on the quoit in the past, who must have researched it as well presumably before passing comment or putting stuff in print.
Anyone spending time at the quoit should soon suss out that it was very unlikely to have looked like it is now on first build but they seem to have gone down the same path as others before them and just copied each others work and didn't seem to be prepared to make claims which they couldn't substantiate for fear of being made to look foolish.
I on the other hand can say what I like and am prepared to be challenged once my Tome is out. But I can promise you that I believe I have made a good fist of it and won't be ridiculed through lack of careful consideration. If I'm wrong I'm wrong but people will always have a chance to come up with their own ideas which of course will also be open to question.

Maybe anyone who has written about it in the past didn't believe that it has been modified since first build . If they had believed so there was nothing to stop them mentioning it either .

Without excavation to show that there has been some modification to show that there may have been some modification I don't see any reason to doubt the standard model which is also the most parsomonious . The back stone slipped and the northern ante chamber stone is missing .There is even a local precedent for the uncommon ante chamber at Zennor .